NOR Gate from Transistors
Build a NOR gate using transistors—the output is HIGH only when both inputs are LOW. NOR is a universal gate that can create any other logic function!
Overview
The NOR gate is "NOT OR"—it outputs HIGH only when all inputs are LOW. If any input is HIGH, the output goes LOW. NOR gates are special because they're "universal"—you can build any other logic gate using only NOR gates! This makes them fundamental to computer architecture.
Components Needed
- 2x Switch
- 4x 10kΩ resistor
- 1x LED
- 3x 2N2222 transistor
- severalx Wires
Instructions
Gather Your Components
You'll need two switches, three 2N2222 transistors (two for OR, one for NOT), four 10kΩ resistors, an LED, wires, breadboard, and 5V power.
Build the NOR Gate Circuit
The NOR gate can be built with parallel transistors sharing a pull-up resistor. When either input is HIGH, the corresponding transistor pulls the output LOW. Only when both transistors are OFF does the output get pulled HIGH by the resistor.
Test: Both OFF → LED ON
With both switches OFF, neither transistor conducts. The pull-up resistor pulls the output HIGH, turning the LED ON. This is the only combination where NOR outputs HIGH.
Test: Left ON, Right OFF → LED OFF
Turn the left switch ON. The left transistor conducts and pulls the output LOW, turning the LED OFF. Any HIGH input produces a LOW output.
Test: Left OFF, Right ON → LED OFF
Switch positions—left OFF, right ON. The right transistor conducts, again pulling the output LOW. The LED is OFF.
Test: Both ON → LED OFF
Turn both switches ON. Both transistors conduct, definitely keeping the output LOW. The LED remains OFF. NOR only outputs HIGH when BOTH inputs are LOW.
Challenges
- Build a NOT gate using only NOR gates (hint: tie both inputs together)
- Build an OR gate using only NOR gates
- Build an AND gate using only NOR gates
- Research why early computers (like Apollo Guidance Computer) used only NOR gates
- Design a simple latch (memory) using two NOR gates with cross-feedback